Current automotive designs incorporate both steel and molded polymeric component parts. Typically, the metal components are made of stamped sheet steel. Major polymeric body components are compression molded polyester sheet molding compound (SMC) or reaction injection molded (RIM) urethane. A preferred method of joining such steel and polymeric parts to each other is to use a rapidly curable but strong adhesive. Generally, bond strengths of at least 25,000 kiloPascals are required for joining steel. Acrylic and polyurethane adhesives are rapidly curable and initially provide such bond high strengths. However they cannot generally tolerate paint oven temperatures of 180.degree. C. and higher encountered when the final paint coat is applied.
Bond strengths greater than 25,000 kiloPascals are also attainable with two-part epoxy compositions. In a two-part system, the epoxide resin and curative constituents are separately measured out and mixed immediately before use. Such measuring and mixing make them undesirable for assembly line operations.
In order to take advantage of the bond strength of epoxies but to avoid the measuring and mixing steps, considerable effort has been directed towards the development of a suitable one-part epoxy adhesive. This necessarily entails the use of a latent catalyst or curative that is substantially inert to epoxy cure reactions at room temperature but which will cure rapidly at elevated temperatures.
We sought to develop a one-part epoxy adhesive for assembly line use. The criteria it had to meet included a shelf life at room temperature (about 25.degree. C.) of 30 to 90 days, a cure in four minutes or less at a temperature between about 130.degree. and 150.degree. C. and an ability to withstand exposure to temperatures up to about 200.degree. C. without substantial loss of bond strength. As a practical matter, these properties of long shelf life, high temperature resistance and rapid cure are mutually exclusive. Improving one of them causes a deterioration of one or both of the others.
For example, dicyandiamide ##STR1## or DICY is known to be an excellent latent curing agent or catalyst for epichlorohydrinbisphenol A (DGEBA) based epoxy resins. However, temperatures well over 150.degree. C. are needed to activate the DICY and effect cure in a matter of a few minutes. The addition of an imidazole salt to a DICY-containing composition is known to lower its cure temperature and increase its cure rate, but it also lowers bond strength.